In the control of fluid in industrial processes, such as oil and gas pipeline systems, power plants, chemical processes, etc., it is often necessary to reduce the pressure of a fluid. Adjustable flow restriction devices, such as flow control valves and fluid regulators, and other fixed fluid restriction devices, such as diffusers, silencers, and other back pressure devices, are utilized for this task. The purpose of the fluid control valve and/or other fluid restricting device in a given application may be to control fluid rate or other process variables, but the restriction induces a pressure reduction inherently as a by-product of its flow control function.
Pressurized fluids contain stored mechanical potential energy. Reducing the pressure releases this energy. The energy manifests itself as the kinetic energy of the fluid—both the bulk motion of the fluid and its random turbulent motion. While turbulence is the chaotic motion of a fluid, there is momentary structure in this random motion in that turbulent eddies (or vortices) are formed, but rapidly break down into smaller eddies which in turn also break down, etc. Eventually viscosity damps out the motion of the smallest eddies and the energy has been transformed into heat.
Pressure and velocity fluctuations are associated with the turbulent fluid motion that act upon the structural elements of the piping system, causing vibration. Vibration may lead to fatigue failure of pressure retaining components or other types of wear, degradation of performance, or failure of attached instruments. Even when not physically damaging, vibration generates air-borne noise that is annoying to or may damage the hearing of people.
In industrial applications involving liquids, the chief source of noise, vibration, and damage from the pressure reduction of liquids is cavitation. Cavitation is caused in a flow stream when the fluid passes through a zone where the pressure is below its vapor pressure. At this reduced pressure, vapor bubbles form and subsequently collapse after traveling downstream into a zone where pressure exceeds the vapor pressure. The collapsing vapor bubbles may cause noise, vibration, and damage. Ideally, therefore, a fluid pressure reduction device would gradually decrease fluid pressure without dropping below the vapor pressure. In practice, however, such a pressure reduction device is overly difficult and expensive to produce, and therefore fluid pressure reduction devices are known that use multiple stages of pressure reduction. The final pressure drop in such devices is relatively small, which may produce less bubbles and less cavitation.
Currently there are available fluid control valves containing a valve trim in the form of stacked disks forming a fluid pressure reduction device. The stacked disks define a plurality of fluid flow passages designed to create a pressure reduction in the fluid.
One device using stacked disks has tortuous fluid flow paths formed therein. In this device, each of the fluid flow passages is designed with a series of consecutive right angle turns so that the fluid flow changes directions many times in a tortuous path as the path traverses from the inlet to the outlet. In such devices, it is intended for each right angle turn to produce a discrete pressure drop, so that the tortuous path produces a multi-stage pressure reduction. In reality, however, it has been found that the intermediate right angle turns in the flow passages do not effectively create a restriction for staged pressure reduction. In addition, the pressure reduction created by the tortuous path is unpredictable since the pressure reduction effected by each right angle turn is not known. Furthermore, it has been found that the right angle turns may generate pressure and mass flow imbalances and flow inefficiency. The pressure imbalances may lead to the creation of low pressure areas within the device where the fluid momentarily drops below the vapor pressure and subsequently recovers, thereby creating cavitation and causing damage. Flow imbalances affect the pressure drop and fluid velocity through the device, wherein a greater mass flows through some passages to result in increased velocity.
In addition, the tortuous path device has passage outlets oriented so that fluid flow exiting the passages converges. As a result, fluid jets exiting the adjacent outlets may collide to form a larger jet flow having greater stream power, thereby increasing the noise level.
The above recited deficiencies and others in currently available trim devices significantly reduce the effectiveness of these devices in providing desired noise attenuation, vibration reduction, and cavitation damage reduction or elimination. Accordingly, it is desired to eliminate the above deficiencies as well as to provide other improvements in the trim devices so as to enable them to provide enhanced noise attenuation characteristics.